In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the conventional multiple access systems for delivering simultaneous services to a certain number of terminals. The basic idea underlying the FDMA and TDMA systems includes dividing the available resource, respectively into several frequencies or into several time slots, such that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception takes place at different frequencies, and also in different time slots. Unlike the systems using frequency division or time division, CDMA systems (code division multiple access) enable multiple users to share a common frequency and a common time channel by using a coded modulation. Among the CDMA systems are the CDMA 2000 system, the WCDMA system (wide band CDMA) and the IS-95 standard.
In CDMA systems, as is well known to those skilled in the art, a scrambling code is associated with each base station and is used to distinguish one base station from another. In addition, an orthogonal code, known to those skilled in the art as the OVSF code, is allocated to each remote terminal (such as a cellular mobile telephone). All the OVSF codes are mutually orthogonal which distinguishes one channel from another. Before transmitting a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the channel.
In CDMA systems, we can still distinguish those that use a distinct frequency for transmission and reception (CDMA-FDD system) from those which use a common frequency for transmission and reception, but distinct time domains for transmission and reception (CDMA-TDD system). The invention applies advantageously to communication systems of the CDMA type, and more particularly, to systems of the WCDMA type with terrestrial radio access (UTRA FDD/TDD).
The incident signal received by a mobile telephone for example comprises different versions delayed in time from the signal initially transmitted. These versions are the result of the multi-path transmission characteristics of the transmission environment between a base station and the telephone. Each path introduces a different delay.
The Rake receiver in a cellular mobile telephone operating in a CDMA communication system is used to carry out temporal alignment, descrambling, compression, channel correction and combination of the delayed versions of the initial signals to deliver the information streams (symbols) contained in the initial signals.
A Rake receiver is usually made up of several fingers. Each finger is designed to demodulate a given path received at a given moment. Baseband demodulation essentially includes descrambling and compressing. Then, the signal, after compression, undergoes a channel correction, which usually includes a phase correction and an amplitude correction. All these operations relating to demodulation are carried out, within each finger, in several units of demodulation respectively associated with different physical transmission channels.
Furthermore, the channel estimate based on the channel corrections which will be made is produced in parallel on a window comprising a predetermined number of symbols of a pilot channel, for example 10 symbols. If the signal in each demodulation unit is not delayed before making the channel correction, then the channel correction for a symbol is made by using an estimate made on only the 10 symbols preceding the symbol.
Also, provision is usually made to delay the symbol before channel correction so as to make the channel correction with the estimate made on the five symbols of the pilot channel preceding the symbol in question and on the next five. In this regard, provision is currently made for one memory per channel, i.e., one memory per demodulation unit in each finger of the receiver. This leads to considerable memory sizes and consequently to a significant surface congestion on the silicon.